1. Field of the Invention
The present invention relates to a lead-free bump and a method of forming the same, and more particularly to a lead-free bump with suppressed formation of voids, obtained by reflowing a plated film of Sn—Ag solder alloy having an adjusted Ag content, and a method of forming the lead-free bump, and also to a plating apparatus for forming such a lead-free bump.
2. Description of the Related Art
In surface mounting technology of semiconductor devices or the like, it is very important to carry out soldering with high reliability. Although an eutectic solder containing lead (Sn:Pb=63:37) has heretofore been used widely in soldering, in the light of environmental contamination and because of the problem of α-rays generation from lead, development of lead-free soldering is under way.
For example, lead-free soldering by means of printing or electroplating is being studied. With printing, however, there is a limit in its approach to fine pitches through the use of a metal mask. Electroplating is therefore becoming mainstream, for example, for the formation of wafer bumps.
In the case of forming wafer bumps by electroplating, a heating operation (reflowing) is usually carried out to make plated films into the form of balls. The reflow temperature is preferably as low as possible in order to avoid thermal damage to other parts that exist in the substrate. From this viewpoint, many developments of solder alloys have been directed to making the composition of an alloy closest possible to the eutectic composition of the alloy in order to make use of the eutectic point.
However, the formation of bumps by electroplating has the problem that upon reflowing of bumps, voids can be formed in the bumps. The formation of voids is particularly marked with bumps of an Sn—Ag alloy, lowering the reliability of the bumps.